The structure of integrated Moloney murine leukemia virus (M-MuLV) DNA sequences will be studied and correlated with the expression of viral genetic information in vivo and in vitro. We will characterize recently isolated recombinant DNA clones carrying integrated M-MuLV, as well as additional clones to be isolated, with respect to possible nucleic acid sequence homologies among flanking cellular DNA sequences and the primary nucleic acid sequence of the cell-virus junction regions. We will develop methods to identify, by very limited DNAse I digestion of infected cell nuclei, individual integrations of M-MuLV DNA sequences which are actively transcribed. We will compare the transcriptional activity of individual integrated M-MuLV's with the ability of these DNA sequences as isolated or recombinant DNA clones, to produce virus infection upon DNA transfection of uninfected cells. We will analyze the organization of the endogenous family of M-MuLV related DNA sequences by restriction enzyme mapping and blot transfer-hybridization and compare these classes of endogenous sequence organization to the DNA organization of individual infectious murine endogenous viruses. We will follow, at the level of viral DNA sequences, the spread of virus infection and the development of lymphomas following in vivo infection of mice with M-MuLV. The DNA of preleukemic and leukemic lymphoid tissues and separated cell types will be analyzed for the presence of M-MuLV sequences, and for viral DNA rearrangements specific to leukemic or preleukemic cells. We will generate in vitro recombinants between cloned M-MuLV and cloned endogenous virus-like sequences and test their biological activity in order to study the roles of adjacent cellular DNA sequences and viral DNA sequences in the expression of virus genetic information.